Replace all hps after 24 months, with the exception of the son agro and super agro, replace after 15 months. Fluorescents should be replaced every 15 months. After 12 months, check the tubes if running more than one tube on a single ballast. Always replace all fluorescent tubes at the same time. Remember to always write down the day that you start using a new HID lamp. This will allow you to calculate when to replace it for best results.